The present invention relates to a system and method for monitoring hemodynamic vascular parameters of a patient and, more particularly, to a system and method utilizing photoplethysmography to monitor parameters associated with, for example, blood pressure and blood flow of a patient.
Hemodynamic vascular parameters such as blood pressure, blood flow and the like which are typically measured using non-invasive procedures are routinely monitored by physicians in order to determine the physiological state of the heart and circulatory system of a patient. Of these hemodynamic vascular parameters, blood pressure is the most commonly monitored.
Blood pressure is the force within the circulatory system of an individual that ensures a flow of blood and delivery of oxygen and nutrients to the tissue.
Abnormal blood pressure readings and/or blood pressure fluctuations over time are oftentimes indicative of heart or circulatory disorders. Hypertension is one of the most common diseases in the adult population, often accompanied by secondary cardiovascular damage. In addition, prolonged reduction or loss of pressure severely limits the amount of tissue perfusion and could therefore result in damage to, or even death of, the tissue. Although some tissues can tolerate hypoperfusion for fairly long periods of time, the brain, heart and kidneys are very sensitive to a reduction in blood flow. Thus, blood pressure is a frequently monitored both routinely and also during surgical procedures where ample supply of blood to tissues is crucial for tissue survival.
During and after surgery, blood pressure is affected by the type of surgery and physiological factors such as the body's response to the surgery. Moreover, during and after surgery, blood pressure is manipulated and controlled using various medications. Often, these physiological factors and the given medications result in a situation requiring immediate blood pressure measurement, and corrective action.
In some clinical situations, dramatic changes in blood pressure can occur instantaneously. For example, a sudden change in pressure may occur due to a reaction to drug therapy. Also, patient reactions to the surgery, sudden occlusion of an artery due to an embolism, or even sudden cardiac arrest are a few of the possibilities. It is very important to detect these sudden changes immediately, and to insure that the direction and amount of the changes be accurate within certain limits. Conversely, it is equally important that false indications of significant blood pressure changes do not occur.
Due to the above described reasons, constant monitoring of blood pressure of a patient is often necessary. The traditional method of measuring blood pressure is with the use of a occlusive cuff, a stethoscope and a pressure manometer. However, this technique is slow, subjective in nature, requires the intervention of a skilled clinician and does not provide the timely readings frequently required in critical situations.
For these reasons, two methods of measuring blood pressure have been developed: invasive, continuous (beat-to-beat) measurements and noninvasive, intermittent methods that use an automated occlusive cuff device.
Invasive methods suffer from several inherent limitations including the risk of embolization, nerve damage, infection, bleeding and vessel wall damage. In addition due to their invasive nature such methods are more suitable to blood pressure monitoring during surgical procedures.
The noninvasive cuff method does not have the inherent disadvantages of the invasive technique, however it also does not provide the continuous beat-to-beat pressure variations obtainable with the invasive method. Further, the noninvasive cuff method typically requires 15 to 45 seconds to obtain a measurement, and since it is an occlusive technique, the method should allow a minimum of 15 seconds to ensure sufficient venous recovery. Thus, at best there is typically 1/2 to 1 minute between updated pressure measurements. When fast acting medications are administered, this is an inordinately long amount of time to wait for an updated pressure reading. Also, frequent cuff inflation over extended periods of time may result in ecchymosis and/or nerve damage in the area underlying the cuff.
Several systems have been developed to address the need for continuous, noninvasive blood pressure measurement.
European Patent Document 0048060 and U.S. Pat. Nos. 4,406,289, 4,510,940 and 4,539,997 to Wesseling et al., U.S. Pat. No. 4,475,554 to Hyndman, U.S. Pat. No. 4,524,777 (1985) to Kisioka, U.S. Pat. No. 4,846,189 to Sun and U.S. Pat. No. 4,869,261 to Penaz, all relate to methods and devices utilizing a technique known as photoplethysmography which is commercially implemented in a device known as the FINAPRES system (Omeda).
The FINAPRES system uses a small inflatable air cuff containing an infrared photoplethysmograph. The cuff is applied to one of the subject's fingers or thumb, and the photoplethysmograph measures the absorption at a wavelength specific for hemoglobin. The device first measures the individual's mean arterial pressure, and then varies the cuff pressure around the finger to maintain the transmural pressure at zero as determined by the photoplethysmograph. The device tracks the intra-arterial pressure wave by adjusting the cuff pressure to maintain the optical absorption constant at all times.
There are several major disadvantages to this technique. The signal amplitude detected by the photoplethysmograph is a function of the changes in the diameter of the artery within the finger, and is determined by the compliance characteristics of the artery. The device maintains this amplitude at a constant value. This value, or set point, must correspond to the point of zero transmural stress in order to determine the correct pressure. During surgery for example, the device cannot differentiate between changes in photoplethysmograph amplitude due to intra-arterial pressure changes and those due to arterial wall compliance changes. Consequently, the FINAPRES system cannot accurately respond to pressure changes caused by changes in vasomotor tone. In addition, maintaining continuous cuff pressure causes restriction of the circulation in the finger being used, which is uncomfortable when maintained for extended periods of time such as during surgery or during a stay in an intensive care unit.
U.S. Pat. Nos. 4,669,485, 4,718,426, 4,718,427 and 4,718,428 all to Russel, describe a device using a conventional blood pressure cuff applied to a person's upper arm to sense an oscillometric signal. The subject's blood pressure is obtained initially by the oscillometric technique, and then changes in the oscillometric signal indicate changes from this initial reference pressure.
There are two inherent limitations to this device. First, the use of a large air bag as the sensing device provides a means for detecting the fundamental and lower harmonics of the blood pressure signal (up to a few Hertz), but also acts to attenuate many higher order harmonics containing key information relating to blood pressure variations. Second, the use of a cuff to detect the oscillometric signal creates a signal that is very sensitive to patient movement. Since patient movement is often encountered during surgery or in critical care situations, the device requires frequent recalibration to be accurate.
U.S. Pat. Nos. 4,269,193, 4,799,491 and 4,802,488 to Eckerle, U.S. Pat. No. 4,423,738 to Newgard, and U.S. Pat. No. 5,165,416 to Shinoda et al., all describe methods and devices for detecting the pressure wave in the underlying artery of an individual using a technique known as the tonometric technique.
These device and methods utilize a multi-element piezoresistive detector to noninvasively detect the blood pressure wave at the radial artery. This signal is then processed and changes in its amplitude are used to interpret changes to the pressure values obtained using the conventional oscillometric technique.
A major drawback to this technique lies in the method of interpreting changes to the waveform signal. Reliance solely on amplitude changes is misleading since the signal amplitude may increase or decrease with an increase in blood pressure, etc. Secondly, it is dependent on the artery being exactly flat, and variations in artery flatness can introduce errors. It also assumes that the selected sensing element is small with respect to the artery, and that it does not move from its position centered over the artery. Thus, any movement such as that often encountered in surgery or critical care situations will reduce the accuracy of this device.
European Patent Document 0 443 267 A 1 to Smith, describes a technique for monitoring changes in pulse transit time to provide a continuous, noninvasive measure of blood pressure. This technique was developed by Sentinel Monitoring, Inc., of Indianapolis, Ind., and uses a duplicity of photometric detectors similar to those used with oximeters. Typically, one detector is applied to the subject's ear lobe, and the other to a finger. The detectors are used for determining changes in the arrival time of the pulse at each of these sites, and to determine changes in local blood volume. Following an initial calibration pressure measurement obtained with a conventional blood pressure cuff, the Smith device adjusts these pressures by interpreting changes in the pulse transit time and in the optical density of the photoplethysmograph signal.
There are two disadvantages to the Smith technique. First, changes in pulse transit time are very small along major arteries. As a result, small errors caused by patient movement or noise render questionable data. Second, small variations in photoplethysmographic waveform morphology or detector noise can generate measurement errors greater than the sensitivity of the technique to changes in blood pressure.
U.S. Pat. No. 4,960,128 to Gordon, et al., describes a method of determining blood pressure by measuring a single harmonic of the frequencies and displacements of the patient's arterial wall. In Gordon, initial (absolute) blood pressure values arc obtained with a cuff and stethoscope or via an intermittent automated cuff machine, and manually entered into the device as initial reference values. A continuous detector signal is supplied by a noninvasive detector attached to the patient's skin above an artery. The detector signal is filtered, amplified and then sampled. This time sampled detector data is then Fourier transformed into the frequency domain and normalized.
As blood pressure changes, the reported frequencies and their relative amplitudes change. A comparison is made between the fundamental frequency of the present signal and the initial signal. For each shift in frequency (+or -) of 1 Hz, the offset is adjusted correspondingly to yield a change of 5 mm Hg. Thus, Gordon shows a device in which the patient's blood pressure is determined based on the difference in position of the fundamental frequency of the detector signal and initial signal.
The technique described by Gordon does not adequately account for the plurality of factors that can reflect a change in blood pressure. There is a multitude of waveshapes that can accompany a given set of blood pressure values, and the Gordon technique is limited by its function of comparing the frequency with the maximum amplitude of the current signal to that of the initial signal to determine blood pressure.
There is thus a widely recognized need for, and it would be highly advantageous to have, a system and method for accurately and noninvasively monitoring continuous beat-to-beat blood pressure and other important hemodynamic vascular parameters of a patient which is devoid of the above limitations.